These results are more compatible with a deuterostomic rather than protostomic (blastopore forms the mouth) or amphistomic (mouth and anus are formed simultaneously) mode of development in the last common bilaterian ancestor. Thereby, gastrulation in the conservatively evolving protostome P. caudatus follows strictly a deuterostomic pattern. We show that the blastopore gives rise to the anus at the vegetal pole and that the hindgut markers brachyury and caudal are expressed in the blastopore and anus, whereas the foregut markers foxA and goosecoid are expressed in the mouth in the animal hemisphere. Here, we characterize gastrulation and the embryonic expression of genes involved in bilaterian foregut and hindgut patterning in Priapulus caudatus. In protostome development, the mouth forms from the second embryonic opening, not from the blastopore. Priapulids have a conservative morphology, an abundant Cambrian fossil record, and a phylogenetic position that make them a key group in understanding protostome evolution.
The blastopore fate of the bilaterian ancestor plays a crucial role in understanding the transition from radial to bilateral symmetric organisms. In deuterostome, the blastopore becomes the anus. In deuterostomes (the group that includes vertebrates and echinoderms), the blastopore develops into the anus. In protostome, the blastopore becomes the mouth. In protostomes, the blastopore develops into the mouth, and the opposite cavity develops into the anus. During maturation of some animals, it evolves into either anus or the mouth.
This variability, combined with an absence of information from key taxa, hampers the reconstruction of the ancestral developmental mode of the Protostomia and the Bilateria. Solution Verified by Toppr Correct option is C) The opening of gastrula by which an organism communicates with the external environment is called as blastopore. In deuterostomes, the blastopore forms the anus, but its fate in protostome groups is variable. The fate of the blastopore during development in the bilaterian ancestor is currently not well understood.
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